Hydrogenated nitrile butadiene rubber (HNBR) is a valuable elastomer known for its combination of unique properties, including high tensile strength, resistance to abrasion, high oil resistance and resistance to oxidation. There is an increasing demand for elastomers that display these valuable properties at low temperatures. Car manufacturers are requesting elastomers that can be used over a temperature range from −40° C. to +155° C. For aerospace applications elastomers are sought whose working temperature range extends down to −60° C. or even −70° C.
There are known HNBR's that are useful at low temperature. There are commercially available HNBR's that have good low temperature properties. Thus, Therban® XN 535C is a terpolymer, available from Bayer, composed of 21% acrylonitrile, acrylate, and butadiene, that has a residual double bond content (RDB) of 5.5% and a glass transition temperature (Tg) of −38° C. Therban® VP KA 8798 is similar, but differs in having an RDB of less than 0.9%, and, again, has a Tg of −38° C.
European Patent No. 471,250 of Bayer AG, issued February 1992, the disclosure of which is incorporated herein by reference, discloses hydrogenated butadiene/isoprene/(meth)acrylonitrile copolymers. The patent teaches that isoprene-modification of HNBR improves compression set at low temperature (for 97.1 to 98.6% hydrogenation, i.e. 2.9 to 1.4% RDB). The patent discloses copolymers containing 3.5 to 22% by weight of copolymerized isoprene and 18 to 50% by weight of copolymerized acrylonitrile or methacrylonitrile, and having a degree of hydrogenation, based on the C═C double bonds of the polymer, of at least 85%, that is, an RDB not greater than 15%. The examples used copolymers with acrylonitrile contents of 34.1%, 33.3% and 33.5% and isoprene contents of 4.1%, 7.9% and 12.0%, respectively, the balance in each case being butadiene. The degrees of hydrogenation of isoprene were 86.3%, 86.6% and 82.9%, respectively.